1. Field of the Invention
The present invention relates to a running control system for a vehicle.
2. Description of the Related Art
Radar devices that use a laser beam as a detection medium may affect eyesight of pedestrians or the like if the laser beam is applied to their eyes for a predetermined time or more. For this reason, a device that reduces a transmission output of a laser beam while a vehicle is stopped is disclosed in Japanese Patent No. 3261345.
In some countries, it is mandatory to reduce a transmission output of a laser beam while a vehicle has stopped. Also, even without the legal regulations of such countries being a concern, cooling of a radar device with running air cannot be accomplished while the vehicle is stopped. Thus, the radar device may overheat and have a reduced durability while the vehicle is stopped unless a transmission output of the laser beam is reduced to be lower than when operating during a normal state.
However, for a vehicle having a congestion following system which starts and stops the subject vehicle according to the starting and stopping of a preceding vehicle in heavy traffic, if a transmission output of a radar device is reduced during stoppage of the subject vehicle, the radar device with the reduced transmission output is prone to lose sight of the preceding vehicle when a stopped preceding vehicle starts moving again and increases a distance between the subject vehicle and the preceding vehicle, which makes smooth following running control of the subject vehicle toward the preceding vehicle difficult.
The aforementioned drawback in the related art system will be described in detail with reference to FIGS. 10A-10E. As shown in FIG. 10A, an output of the radar device of the subject vehicle is in a power-up mode until the subject vehicle approaches a stopped preceding vehicle and stops. The output of the radar device then enters a power-down mode when the subject vehicle stops, as shown in FIG. 10B. From this state, when the preceding vehicle starts to move again, as shown in FIG. 10C, the preceding vehicle moves beyond a short detectable range of the radar device while in the power-down mode, as shown in FIG. 10D. Thus, even if a driver of the subject vehicle slightly delays operating a start switch for the following running control, the subject vehicle may lose sight of the preceding vehicle, as shown in FIG. 10E, and prevent the radar device from switching to the following running control.